The present invention relates generally to measuring current from device under test and more specifically to correcting current data of a power device under test using a current sensor.
A current sensor such as a Rogowski coil, air-cored coil, etc. is suitable for detecting large currents because of its flexibility. A large current is sometime carried by a thick metal bus bar. If the metal bus bar is intricately wired, it would be difficult to locate a desired line under test of the bus bar at a position for detecting a current through the line with a current sensor. A flexible Rogowski coil allows a user to make a loop around the line under test.
FIG. 1 is a schematic block diagram of a current sensor 22 having a Rogowski coil 12 and an integrator circuit 20 usable in a current probe. The Rogowski coil 12 has a detecting coil 11 and a conducting return wire 9 that are made of a wire. A portion of the wire is formed in loops to produce the detecting coil 11. The return wire 9 has one end 9b connected to the loop end 12b of the detecting coil 11 and is folded back through the center of the detecting coil 11 to the loop beginning end 12a of the detecting coil 11. The loop beginning end 12a of the detecting coil 11 and the free end 9a of the return wire 9 are closely located. In addition, when a user conducts a measurement, the ends 12a and 12b of the detecting coil 11 are arranged to be physically close together so that the detecting coil 11 constitute a magnetic closed loop around a line under test 10. The loop beginning end 12a of the detecting coil 11 is coupled to the integrator circuit 20 that typically has a resister 14, a capacitor 16 and an operational amplifier 18. The free end 9a of the conducting return wire 9 is coupled to ground.
A current Ip flowing in the line under test 10 generates magnetic flux that induces a voltage in the Rogowski coil 12. If the frequency of the current Ip becomes higher, the induced voltage also becomes higher. The integrator circuit 20 maintains a flat frequency characteristic by lowering the gain of the integrator circuit 20 as the frequency of the current Ip increases. U.S. Pat. No. 7,598,724 by Howell et al. and U.S. Pat. No. 6,885,183 by Kato disclose some applications of a Rogowski coil.
FIG. 2 is a partial schematic diagram of a device under test showing the use of Rogowski coil sensor 70 for measuring a current signal. The device under test in FIG. 2 is an IGBT (Insulated Gate Bipolar Transistor) 40 in inverter circuitry for driving an inductive load, or a three-phase motor 46. The inverter circuitry has a plurality of IGBTs 30, 32, 34, 36, 38, and 40 which have respective flywheel diodes 50, 52, 54, 56, 58, and 60. Since the three-phase motor 46 is an inductive load, it stores energy in the inductance and regenerates a flywheel current that passes through the flywheel diodes 50, 52, 54, 56, 58, and 60. A power supply 48 provides power to the inverter circuitry. A PWM (Pulse Width Modulation) controller 44 is coupled to an IGBT driver 42 via a bus to provide a PWM control signal. The IGBT driver 42 provides gate drive voltages to the IGBTs 30, 32, 34, 36, 38, and 40. The pulse width of the gate drive voltages is modulated according to the PWM control signal.
The positive and negative inputs of a differential probe 62 are coupled to the collector and emitter of the IGBT 40 respectively to detect a voltage, Vce, between the collector and emitter. The output of the differential probe 62 may be connected to a first channel (CH1) of a digital oscilloscope 66. The positive and negative inputs of a differential probe 64 are coupled to the gate and emitter of the IGBT 40 respectively to detect a voltage, Vge, between the gate and emitter or a gate drive voltage. The output of the differential probe 64 may be connected to a third channel (CH3) of the oscilloscope 66. A current probe 68 may use a Rogowski coil 70 and integrator circuit 72 to detect an emitter current Ie of the IGBT 40. The output of the current probe 68 is coupled to a second channel (CH2) of the oscilloscope. The digital oscilloscope 66 receives the current and voltage signals from the device under test and stores the signals as digital data for display as waveforms.
FIG. 3 shows voltage and current waveforms of the three channels of the oscilloscope 66. The horizontal and vertical axes are respectively amplitude and time. The time axis of display area A is longer than that of display area B. That is, the voltage and current waveforms of the display area B are zoomed-in version of the voltage and current waveform portions indicated by a box 74 in the display area A. Each pulse of the voltage and current waveforms of CH1 and CH2 shows a surge at the rising edge.
FIG. 4 shows the voltage and current waveforms of the three channels that corresponding to the display area A of FIG. 3 wherein the vertical axis is enlarged relative to that of the display area A of FIG. 3. The current waveform of the second channel shown in FIG. 4 has positive half-cycles and negative half-cycles. The current waveform shows current flowing in the emitter of IBGT 40 during the positive half cycle as a result of the gate drive voltage turning IBGT 40 On and Off using pulse width modulation. During the negative half-cycle, current is drawn through the flywheel diode 60 to the motor 46.
Referring again to the current waveform of the second channel of FIG. 4, an arrow 76 indicates zero ampere level of the current waveform at the left end of the waveform in the display area. FIG. 4 shows that the zero ampere level of the current waveform fluctuates because the integration process in the integrator circuit 72 is not ideal. The fluctuation leads to a measurement error of power. For example, referring to FIG. 3, when the gate drive voltage is low, the IGBT 40 is off and then the collector-emitter voltage Vce is high and the emitter current Ie should be zero. That is, when the IGBT 40 is off, the power loss of the IGBT, or Vce×Ie should be zero. However, the power loss Vce×Ie may shows some value due to the fluctuation error.
AC type current probes, such as a current probe using a Rogowski coil cannot detect DC components in a current signal. What is needed is an apparatus and method that can cancel the fluctuation components at a zero ampere level even though the DC component cannot be detected.